Methods for amplifying nucleic acids provide useful tools for the detection of human pathogens, detection of human genetic polymorphisms, detection of RNA and DNA sequences, for molecular cloning, sequencing of nucleic acids, and the like. In particular, the polymerase chain reaction (PCR) has become an important tool in the cloning of DNA sequences, forensics, paternity testing, pathogen identification, disease diagnosis, and other useful methods where the amplification of a nucleic acid sequence is desired. See e.g., PCR Technology: Principles and Applications for DNA Amplification (Erlich, ed., 1992); PCR Protocols: A Guide to Methods and Applications (Innis et al., eds, 1990).
PCR permits the copying, and resultant amplification, of a target nucleic acid. Briefly, a target nucleic acid, e.g. DNA, is combined with a sense and antisense primers, dNTPs, DNA polymerase and other reaction components. See Innis et al. The sense primer can anneal to the antisense strand of a DNA sequence of interest. The antisense primer can anneal to the sense strand of the DNA sequence, downstream of the location where the sense primer anneals to the DNA target. In the first round of amplification, the DNA polymerase extends the antisense and sense primers that are annealed to the target nucleic acid. The first strands are synthesized as long strands of indiscriminate length. In the second round of amplification, the antisense and sense primers anneal to the parent target nucleic acid and to the complementary sequences on the long strands. The DNA polymerase then extends the annealed primers to form strands of discrete length that are complementary to each other. The subsequent rounds serve to predominantly amplify the DNA molecules of the discrete length.
A variety of factors can lead to non-functional PCR or other amplification reactions. For example, endonucleases may cleave the primers used to amplify a polynucleotide. In such an event, the user is unable to distinguish between the lack of an amplification product resulting from the absence of the appropriate template or a false negative result caused by primer cleavage. The present invention addresses this and other problems.